Method of forming an aluminum oxide layer on anode foil for aluminum electrolytic capacitor

ABSTRACT

A method of producing an oxide layer on a foil for use in a capacitor includes immersing the foil in a first solution; maintaining a target current between the immersed anodic foil and the first solution until a first target voltage is reached to form an oxide layer overlying the foil; maintaining the target current between the immersed foil and the first solution until a second target voltage is reached to reform the oxide layer; removing the foil from the first solution and heating the foil; immersing the heat treated foil in a second solution; maintaining the target current between the immersed foil and the second solution until a third target voltage is reached; and discharging the immersed foil after each of the first, second, and third target voltages are reached.

FIELD OF THE INVENTION

The present disclosure relates generally to the field of electrolyticcapacitors and batteries.

BACKGROUND

Compact, high voltage capacitors are utilized as energy storagereservoirs in many applications, including implantable medical devices.These capacitors are required to have a high energy density, since it isdesirable to minimize the overall size of the implanted device. This isparticularly true of an Implantable Cardioverter Defibrillator (ICD),also referred to as an implantable defibrillator, since the high voltagecapacitors used to deliver the defibrillation pulse can occupy as muchas one third of the ICD volume.

Implantable cardioverter defibrillators, such as those disclosed in U.S.Pat. No. 5,131,388, the disclosure of which is hereby incorporatedherein by reference, typically use two electrolytic capacitors in seriesto achieve the desired high voltage for shock delivery. For example, anICD may utilize two 350 to 400 volt electrolytic capacitors in series toachieve a voltage of 700 to 800 volts.

Electrolytic capacitors are used in ICDs because they have the mostnearly ideal properties in terms of size, reliability and ability towithstand relatively high voltage. Conventionally, such electrolyticcapacitors include an etched aluminum foil anode, an aluminum foil orfilm cathode, and a kraft paper or fabric gauze separator impregnatedwith a solvent-based liquid electrolyte interposed between the anode andthe cathode. While aluminum is the preferred metal for the anode plates,other metals such as tantalum, magnesium, titanium, niobium, zirconiumand zinc may be used. A typical solvent-based liquid electrolyte may bea mixture of a weak acid and a salt of a weak acid, preferably a salt ofthe weak acid employed, in a polyhydroxy alcohol solvent. Theelectrolytic or ion-producing component of the electrolyte is the saltthat is dissolved in the solvent. The entire laminate is rolled up intothe form of a substantially cylindrical body, or wound roll, that isheld together with adhesive tape and is encased, with the aid ofsuitable insulation, in an aluminum tube or canister. Connections to theanode and the cathode are made via tabs. Alternative flat constructionsfor aluminum electrolytic capacitors are also known, comprising aplanar, layered stack structure of electrode materials with separatorsinterposed therebetween, such as those disclosed in the above-mentionedU.S. Pat. No. 5,131,388.

In ICDs, as in other applications where space is a critical designelement, it is desirable to use capacitors with the greatest possiblecapacitance per unit volume. Since the capacitance of an aluminumelectrolytic capacitor is provided by the anodes, a clear strategy forincreasing the energy density in the capacitor is to minimize the volumetaken up by the separators and cathodes and maximize the number ofanodes. A multiple anode stack configuration requires fewer cathodes andpaper separators than a single anode configuration and thus reduces thesize of the device. A multiple anode stack consists of a number of unitseach consisting of, in series, a cathode, a paper separator, two or moreanodes, a paper separator and a cathode, with neighboring units sharingthe cathode between them, all placed within the capacitor case.

The energy density of aluminum electrolytic capacitors is directlyrelated to the surface area of the anodes generated in theelectrochemical etching processes. Typical surface area increases are 40to 1 and represent 30 to 40 million tunnels/cm². An electrochemicalwidening step is used to increase the tunnel diameter after etching toinsure the oxide layer described below will not close off the tunnels.

The high surface area foil is put through an oxidation process to grow avoltage supporting oxide layer with low leakage current and lowdeformation properties. An oven depolarization process is used after theoxidation process to drive off the waters of hydration, induce stresscracking and expose weak areas. A subsequent oxidation process, i.e., areformation process, “heals” the stress cracks and improves theresulting leakage current. The number of defects can be reduced witheach subsequent reformation and depolarization process until a constantlevel of defects is achieved.

However, the inventors have discovered that the use of severaldepolarization steps has a negative effect on capacitance anddeformation in an electrolytic capacitor. Accordingly, there is a needin the art for improved oxide formation processes.

BRIEF SUMMARY

Metal foils having oxide layers, devices using the same, and methods ofmaking the same are disclosed herein.

One aspect of the present disclosure relates to a method of producing anoxide layer on an anodic foil for use in a capacitor. The methodincludes immersing an anodic foil in a first solution; maintaining atarget current between the immersed anodic foil and the first solutionuntil a first target voltage is reached to form an oxide layer overlyingthe anodic foil; discharging the immersed anodic foil after the firsttarget voltage is reached; maintaining the target current between theimmersed anodic foil and the first solution until a second targetvoltage is reached to reform the oxide layer a first time; dischargingthe immersed anodic foil after the second target voltage is reached;removing the anodic foil from the first solution and heating the anodicfoil at a temperature sufficient to induce defects in the reformed oxidelayer; immersing the heat treated anodic foil in a second solution;maintaining the target current between the immersed anodic foil and thesecond solution until a third target voltage is reached to reform theoxide layer a second time; and discharging the immersed anodic foilafter the third target voltage is reached.

Another aspect of the present disclosure relates to a device. The deviceincludes a conductive anode; a layer of a barrier oxide disposed on asurface of the conductive anode, wherein the barrier oxide is analuminum oxide having a boehmite phase and a pseudo-boehmite phase, theamount of the boehmite phase being greater than the amount of thepseudo-boehmite phase; a conductive cathode; a separator disposedbetween the anode and the cathode; and an electrolyte disposed betweenthe anode and the cathode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an electronic component inaccordance with an embodiment of the present disclosure.

FIG. 2 depicts a method for producing an oxide layer in accordance withan embodiment of the present disclosure.

FIG. 3 is an SEM image of an aluminum oxide layer that has a predominantboehmite phase.

FIG. 4 is a graph depicting the deformation over time of a capacitorformed in accordance with an embodiment of the present disclosure.

FIG. 5 is an SEM image of an aluminum oxide layer that has a predominantpseudo-boehmite phase.

FIG. 6 is a graph depicting the deformation over time of a capacitorformed by a comparative method.

DETAILED DESCRIPTION

The following detailed description of methods of forming an oxide layerand capacitor designs refers to the accompanying drawings thatillustrate exemplary embodiments consistent with these devices. Otherembodiments are possible, and modifications may be made to theembodiments within the spirit and scope of the methods and systemspresented herein. It will be apparent to a person skilled in therelevant art that the methods and systems can also be employed toproduce porous anode foils for use in a variety of devices andapplications in addition to their use in implantable cardioverterdefibrillators (ICD). Therefore, the following detailed description isnot meant to limit the devices described herein. Rather, the scope ofthese methods and devices is defined by the appended claims.

FIG. 1 is a cross-sectional view of an electronic component 100.Electronic component 100 includes a housing 102 that contains aplurality of cathodes 104 alternating with a plurality of anodes 108,with each cathode being separated from an adjacent anode by a separator106. Each anode 108 includes a dielectric material 110, e.g., an oxidelayer, on or around its outer surface. Dielectric material 110 may be anoxide that is thermally grown on, or deposited onto, the surface ofanode 108. A high-k dielectric material may be used for dielectricmaterial 110. A conductive electrolyte 112 fills the space between eachof the elements within housing 102. Electrolyte 112 may be a polymer orliquid electrolyte as would be understood by one skilled in the art.Example electrolytes include ethylene glycol/boric acid-basedelectrolytes and anhydrous electrolytes based on organic solvents suchas dimethylformamide (DMF), dimethylacetamide (DMA), orgamma-butyrolactone (GBL). The plurality of cathodes 104 may beelectrically connected to a single, common cathode terminal, and theplurality of anodes 108 may be similarly connected to a single, commonanode terminal.

Electronic component 100 may be, for example, an electrolytic capacitoror a battery. When electronic component 100 is used as a capacitor,example materials for the plurality of cathodes 104 include aluminum,titanium and stainless steel, while example materials for the pluralityof anodes 108 include aluminum and tantalum. When electronic component100 is used as a battery, example materials for the plurality ofcathodes 104 include silver vanadium oxide, carbon fluoride, magnesiumoxide, or any combination thereof, while example materials for theplurality of anodes 108 include lithium metal.

Separators 106 may be provided to maintain a given separation betweeneach cathode 104 and an adjacent anode 108 within housing 102.Additionally, separators 106 may be provided to prevent arcing betweencathode 104 and anode 108 in spaces where dielectric 110 may be verythin or nonexistent, and/or where a void within electrolyte 112 existsbetween cathode 104 and anode 108.

The dielectric layer 110, e.g., an oxide layer, provides a highresistance to current passing between the electrolyte and the anode 108in the capacitor. This current is referred to as the leakage current. Ahigh leakage current can result in poor performance and reliability ofan electrolytic capacitor. In particular, high leakage current resultsin a greater amount of charge leaking out of the capacitor once it hasbeen charged.

In an embodiment, the dielectric layer 110 includes aluminum oxide,where the aluminum oxide includes a pseudo-boehmite phase and a boehmitephase. As discussed in method 200 below, method steps are performed tomaximize the presence of the boehmite phase in the aluminum oxide. Theincreased presence of the boehmite phase can result in less deformationof the capacitor and a reduced maintenance cycle.

The amount of the boehmite phase in the aluminum oxide layer may beabout 50 weight percent (wt %) or greater. In one embodiment, the amountof boehmite phase may range from about 50 wt % to about 51 wt % of thealuminum oxide. The amount of the pseudo-boehmite phase may be about 50wt % or less of the aluminum oxide. In one embodiment, the amount ofpseudo-boehmite phase may range from about 49 wt % to about 50 wt %. Theboehmite phase is less porous than the pseudo-boehmite phase, and mayprovide increased protection against the aqueous solvent of theelectrolyte which can cause deformation of the capacitor. Thepseudo-boehmite phase may be characterized by a higher water contentthan the boehmite phase. While not wanting to be bound by any particulartheory, the inventors believe that the structure of an aluminum oxidelayer having a pseudo-boehmite phase may be a core-shell structure inwhich the boehmite phase is present at the surface and thepseudo-boehmite phase is deeper in the aluminum oxide.

It should be understood that the various elements and dimensions ofelectronic component 100 are not drawn to scale. Although each cathode104, separator 106, and anode 108 are illustrated as being spaced apartfrom one another for the convenience of illustration and labeling, itwould be understood by one skilled in the art that such elements mayalso be stacked together in close physical contact with one another.

FIG. 2 provides an exemplary method 200 of producing an oxidation layeron an anode foil for use in a capacitor. Method 200 begins with a metalfoil having tunnels or pores etched therein. In one embodiment, themetal foil is an aluminum foil, and the oxidation layer formed thereonis an aluminum oxide (Al₂O₃) layer. After the tunnels or pores areformed by an etching process, the tunnels can be widened to furtherincrease the surface area of the etched metal foil. Examples of theetching and widening processes may be found in U.S. Pat. Nos. 6,858,126,6,802,954, and 8,535,507, the disclosures of which are incorporatedherein by reference. Etching and widening processes that produce tunnelsor pores in a metal foil are not required for using the metal foil as ananode within a capacitor. However, the presence of tunnels or pores inthe metal foil drastically increases the surface area of the anode andtherefore the capacity and charge density of the capacitor. The etchedmetal foil may be anodized prior to formation of the oxide layer, forexample, by methods described in U.S. Pat. No. 7,175,676, the disclosureof which is incorporated herein by reference.

The method 200 may have advantages over a conventional method which usesa foil roll, where, for example, a leading section of the foil rollhaving an oxide layer is heated in an oven, while a lagging section ofthe foil roll undergoes an oxide formation process. The method 200 mayuse individual foil sheets instead of a foil roll. Among the advantagesof individual foil sheets as compared to a foil roll is that eachindividual foil can be bled down to a desired leakage current, whereasin a foil roll the leakage current is estimated based on the time aportion of the foil roll spends in a solution to grow or reform an oxidelayer. Because a process using an individual foil sheet can be moretightly controlled, variability in leakage current, capacitance, anddeformation of the capacitor can be reduced.

At step 202, a hydration layer is formed over the exposed metal on theetched metal foil. The etched metal foil may be placed into a bathcontaining water. In one example, the water is de-ionized. The bath ofwater may be held at a temperature of between about 60° C. and about100° C., and preferably at about 95° C. The etched metal foil may remainwithin the bath for between about 2 minutes and about 20 minutes to formthe hydration layer. In an embodiment, the bath of water may besonicated at either sonic or ultrasonic frequencies.

At step 204, an oxide layer is formed on the exposed metal surfaces ofthe etched metal foil. The oxide layer is formed on one or both surfacesof the metal foil by placing the foil into a first solution and applyinga voltage between the metal foil and the first solution.

The composition of the first solution includes an ionogen. Exemplaryionogens can include dimethyl amine sebacate (DMAS) and citric acid. Theionogens may be present in the first solution in amounts between about0.1 wt % and about 2 wt % based on the total weight of the firstsolution, where the total weight includes a solvent, the ionogen, andany other additives. In one embodiment, the amount of ionogen in thefirst solution may be about 0.5 wt %. In one embodiment, the firstsolution includes 0.5 wt % citric acid as an ionogen agent in an aqueoussolvent. The first solution may also include a phosphate. The phosphatecan aide in the creation of a boehmite phase in the oxide layer withoutdamaging the leakage current of the oxide layer. When present in thefirst solution, the phosphate may constitute between about 5 ppm andabout 20 ppm of the solution. Exemplary phosphates may include anyphosphate salt. The first solution may be maintained at a temperatureranging from about 80° C. to about 100° C. during step 204. In oneembodiment, the temperature of the first solution is maintained at about85° C.

In operation, a device, such as a power supply that can be set tomaintain a constant current and/or a constant voltage, is used to raisean applied voltage until the current between the metal foil and thefirst solution reaches a target current. The target current ranges fromabout 2000 mA to about 5000 mA. When normalized to the surface area ofthe foil, the target current ranges from about 7.4 milliAmps per squarecentimeter (mA/cm²) to about 18.5 mA/cm². As the oxide layer growsand/or reforms, the resistance within the circuit increases.Accordingly, to maintain the target current, the applied voltage isadjusted until a first target voltage is reached. The first targetvoltage may be between about 430V and about 510V. Once the first targetvoltage is reached, the first target voltage is maintained and thecurrent is allowed to drop as the oxide layer continues to grow and/orreform and the resistance continues to increase. When the current dropsto a desired level, step 204 can be terminated. The desired level, i.e.,the desired leakage current, can be between about 0.1 mA/cm² and about2.0 mA/cm². In one embodiment, the desired leakage current is about 1.1mA/cm². In some embodiments, the formation process at step 204 cancontinue for up to about 3 hours before reaching the desired leakagecurrent.

As the desired leakage current is reached, the voltage may be dropped,for example, to about 50 V. The current may continue to drop as thevoltage is lowered until the foil is discharged to 0 V.

The thickness of the oxide layer formed on the metal foil isproportional to the first target voltage. In some embodiments, a layerthickness of between about 10 Angstroms and about 15 Angstroms perapplied volt may be achieved.

At step 206, a first reformation process is performed. The firstreformation process may occur about 5 to 20 minutes after step 204 iscompleted. In some embodiments, the first reformation process occursabout 5 minutes after completion of the formation process at step 204.The first reformation process reforms the oxide layer on the metal foil.In one embodiment, the first reformation process is performed in thefirst solution without having removed the metal foil after step 204.

For steps that use the same solution, such as the formation process atstep 204 and the first reformation process at step 206, the firstsolution may be monitored and may be recharged if it becomes necessary.For example, the first solution can be monitored for conductivity. Ifthe conductivity drops below about 500 microsiemens (μS), ionogen can beadded to the solution to increase the conductivity.

The first reformation process is similar in operation to the formationprocess at step 204, except that the target voltage, a second targetvoltage, is lower than the first target voltage. In one embodiment, thedifference between the first target voltage at step 204, and the secondtarget voltage at step 206, is about 5 volts. In some embodiments, thedifference in the first and second target voltages may range from about0 V to about 10 V. The second target voltage may be between about 420 Vand about 505 V. As with the formation process at step 204, the targetcurrent may be maintained until the second target voltage is reachedbetween the metal foil and the first solution. The second target voltageis then maintained while the current is allowed to decrease. As theresistance increases due to reformation of the oxide layer, the currentdrops until the desired leakage current is reached. The desired level ofthe leakage current may be between about 0.1 mA/cm² and about 2.0mA/cm². In one embodiment, the desired level of leakage current is about0.74 mA/cm². In some embodiments, the first reformation process maycontinue for up to about 1 hour before the desired leakage current isreached. The foil can then be discharged to 0 V in the same manner asdiscussed above for step 204.

At step 208, the metal foil having the reformed oxide layer thereon isremoved from the first solution and heated to expose weaknesses and/ordefects in the reformed oxide layer. The temperature of the heating stepmay be sufficient to induce defects in the reformed oxide layer. In someembodiments, the foil may be heated to a temperature of between about350° C. and about 650° C. In an embodiment, the foil may be heated to atemperature of about 500° C. The heating may be performed underatmospheric pressure conditions in an oven or furnace for up to about 5minutes. In some embodiments, the heating may be performed for a timeperiod of between about 30 seconds and about 6 minutes. In oneembodiment, the heating is performed for about 5 minutes at about 500°C.

The heating at step 208 may be the only heating step performed duringthe entire process. Optionally, as discussed below, if a desired leakagecurrent is not achieved after a third reformation step, then a secondheating step may be performed.

At step 210, after the oxide layer has been subjected to the heatingstep, a second reformation process is performed by placing the metalfoil into a second solution. The composition of the second solution maybe similar or equivalent to that of the first solution. It is possibleto switch ionogens between the first and second solutions, if desired.The second reformation process is similar in operation to the firstreformation process at step 206, except that the target voltage, a thirdtarget voltage, may be lower than the second target voltage. In someembodiments, the difference between the second target voltage and thethird target voltage may range from about 0 V to about 10 V. In oneembodiment, the third target voltage is lower than the second appliedvoltage by about 5 Volts. The third target voltage may be between about420 V and about 505 V. In the second reformation process, the targetcurrent may be maintained until the third target voltage is reached. Thethird target voltage is then maintained while the current decreases. Asthe resistance increases due to reformation of the oxide layer, thecurrent drops until the desired leakage current is reached. The desiredlevel of the leakage current may be between about 0.1 mA/cm² and about2.0 mA/cm². In one embodiment, the desired level of leakage current isabout 0.56 mA/cm². In some embodiments, the second reformation processmay continue for up to about 40 minutes before the desired leakagecurrent is reached. The foil can then be discharged to 0 V in the samemanner as discussed above for step 204.

At step 212, a third reformation process is performed. In oneembodiment, the third reformation process is performed in the secondsolution without having removed the metal foil after step 210. The thirdreformation process is similar in operation to the second reformationprocess at step 210. In one embodiment, the third reformation process isperformed under the same conditions, i.e., the same target voltage andtarget current, as the second reformation process. In some embodiments,the target voltage at step 212, the fourth target voltage, may bebetween about 420 V and about 505 V. In the third reformation, thetarget current is maintained until the fourth target voltage is reached.The fourth target voltage is then maintained while the currentdecreases. As the resistance increases due to reformation of the oxidelayer, the current drops until the desired leakage current is reached.The desired level of the leakage current may be between about 0.1 mA/cm²and about 2.0 mA/cm². In one embodiment, the desired level of leakagecurrent is about 0.44 mA/cm². In some embodiments, the third reformationprocess may continue for up to about 20 minutes before the desiredleakage current is reached. The foil can then be discharged to 0 V inthe same manner as discussed above for step 204.

If the desired leakage current is not reached during the thirdreformation process at step 212, the metal foil having the reformedoxide layer thereon optionally may be removed from the second solutionand heated. The temperature of the heating step may be sufficient toinduce defects in the reformed oxide layer. In some embodiments, thefoil may be heated to a temperature of between about 200° C. and about400° C. In an embodiment, the foil may be heated to a temperature ofabout 300° C. The heating may be performed under atmospheric pressureconditions in an oven or furnace for up to about 5 minutes. In someembodiments, the heating may be performed for a time period of betweenabout 30 seconds and about 6 minutes. In one embodiment, the heating isperformed for about 5 minutes at about 300° C. After this optional step,the method proceeds to step 214.

At step 214, a fourth reformation process is performed. In oneembodiment, the fourth reformation process is also performed in thesecond solution without having removed the metal foil after step 212.The fourth reformation process is similar in operation to the thirdreformation process at step 212. In one embodiment, the fourthreformation process is performed under the same conditions, i.e., thesame target voltage and target current, as the second and thirdreformation processes. In some embodiments, the target voltage at step214, the fifth target voltage, may be between about 420 V and about 505V. In the fourth reformation process, the target current is maintaineduntil the fifth target voltage is reached. The fifth target voltage isthen maintained while the current decreases. As the resistance increasesdue to reformation of the oxide layer, the current drops until thedesired leakage current is reached. The desired level of the leakagecurrent may be between about 0.1 mA/cm² and about 2.0 mA/cm². In oneembodiment, the desired level of leakage current is about 0.41 mA/cm².In some embodiments, the fourth reformation process may continue for upto about 20 minutes before the desired leakage current is reached. Thefoil can then be discharged to 0 V in the same manner as discussed abovefor step 204.

In an embodiment, the reformation processes at steps 206, 210, 212, and214 may be performed under the same conditions, i.e., the same targetvoltage and same target current. In other embodiments, the targetvoltage can be stepped down between the first reformation process atstep 206 and the fourth reformation process at step 214. In oneembodiment, the step down can be a linear step down, for example, from500 V, to 490 V, to 480 V, to 470 V for steps 206, 210, 212, and 214,respectively. In other embodiments, the step down can be non-linear forsteps 206, 210, 212, and 214, respectively.

Though not wishing to be bound by any particular theory, it is believedthat the optimization of the boehmite phase relative to thepseudo-boehmite phase decreases deformation in the capacitor andmaintains a low leakage current. In the methods disclosed herein, theseimprovements are achieved by using a voltage derating of about 1.15 orhigher. For example, for a capacitor that operates at 425 V, theformation and reformation processes may be performed at at least about490 V, which results in a derating of 490 V/425 V, or 1.15. The methodsdisclosed herein allow for less oven treatments and maximize theboehmite phase while reducing the leakage current.

Once formation and reformation of the oxide layer is complete,additional method steps may be performed prior to assembling the anodefoil having the oxide layer into a device. For example, the metal foilhaving the oxide layer thereon can be further processed to incorporatephosphate. This process can include dipping the metal foil having theoxide layer thereon into a phosphate-containing solution. Thephosphate-containing solution may include about 1 wt % to about 5 wt %of a phosphate compound, based on 100 parts by weight of water.Exemplary phosphate compounds can include ammonium dihydrogen phosphate.In operation, the metal foil may be dipped in the phosphate-containingsolution for about 1 to about 5 minutes and then removed from thesolution and rinsed for about 1 to about 5 minutes.

Other method steps can include, in forming an anode suitable for use ina device, removing a section from the foil having the oxide layer formedthereon. The section may be, for example, mechanically cut, punched, orsheared from the foil, or cut using a laser. The section of foil may besized to fit within the housing of a capacitor. The section of foil willhave exposed metal along one or more edges that were previously attachedto the larger foil.

A hydration layer can be formed over the exposed metal edges of thesection of foil. To do so, the foil section may be placed into a bathcontaining water. In one example, the water is de-ionized. The bath ofwater may be held at a temperature of between about 60° C. and about100° C., and preferably at about 95° C. The foil section may remainwithin the bath for between about 2 minutes and about 20 minutes to formthe hydration layer. In an embodiment, the bath of water may besonicated at either sonic or ultrasonic frequencies. The formation ofthe hydration layer will help to form a better quality oxide during theaging process, and also helps to reduce and/or smooth out the formationof burrs at the edges of the foil section.

The foil section, now having a hydration layer on its edges, is placedinto a separate bath that includes ammonium dihydrogen phosphate to forma passivation layer over the foil edges. Note that this step is notrequired, but will increase the lifetime of the capacitor. In anembodiment, the ammonium dihydrogen phosphate bath is maintained at atemperature of between about 50° C. and about 90° C., and preferably atabout 70° C. The bath contains between about 0.1 wt % and about 5.0 wt%, and preferably about 2.0 wt %, of ammonium dihydrogen phosphate. Thefoil section may be placed in the bath of ammonium dihydrogen phosphatefor between about 1 and about 4 minutes. Afterwards, the foil sectionmay be removed from the bath and rinsed under de-ionized water forbetween about 1 and about 12 minutes.

The foil section having the hydration layer can be assembled within acapacitor as the anode. Any number of such foil sections may be placedinto the capacitor to form a single anode. An electrolyte is added tothe capacitor. In one embodiment, the electrolyte may have a watercontent below about 3.0 wt %. In another embodiment, the electrolyte mayhave a water content between about 0.5 wt % and about 3.0 wt %. Anelectrolyte with a lower water content may be used in capacitors havinganodes with a hydration layer when compared to conventional capacitordesigns. Using an electrolyte with a water content between about 0.5 wt% and about 3.0 wt % may reduce the deformation of the capacitor byabout 10% when compared to electrolytes with higher water contents.

Example 1

In Example 1, an anode having an oxide layer was prepared in accordancewith embodiments of the present disclosure. The anode was assembled intoan electrolytic capacitor and deformation was measured using anaccelerated aging process.

An aluminum foil (270 cm² surface area as a plain sheet, etched tunnelsto increase surface area about 40 times from plain sheet, 115 μmthickness) was placed in a water bath for 8 minutes at 95° C. to form ahydration layer. The aluminum foil was removed from the water bath andplaced in a first aqueous solution having 0.5 wt % citric acid. An oxidelayer was formed on the aluminum foil using a target current of 14.8mA/cm² and a target voltage of 490 V. The formation process continued atthe target current until the target voltage was reached, and then thetarget voltage was maintained and the current was allowed to drop. Thecurrent dropped until the leakage current reached 1.11 mA/cm². The timefor forming the oxide layer was about 160 minutes. The foil wasdischarged to 0 V after the formation process. After formation, a firstreformation of the oxide layer was performed in the first aqueoussolution using a target voltage of 485 V and the same target current asin the formation process. The leakage current after the firstreformation process reached 0.74 mA/cm². The time for the firstreformation process to reach the desired leakage current was about 50minutes. The foil was discharged to 0 V after the first reformationprocess.

After the first reformation process, the anode foil having the reformedoxide layer was removed from the first aqueous solution and heated in anoven (Linberg Blue M, available from Thermo Fisher Scientific, Inc.) atatmospheric pressure conditions for 5 minutes at 500° C.

The heated anode foil was removed from the oven and placed in a secondaqueous solution having 0.5 wt % citric acid. A second reformation ofthe oxide layer was performed at a target voltage of 480 V and the sametarget current as the first reformation process. The leakage currentafter the second reformation process reached 0.56 mA/cm². Theapproximate time for the second reformation process to reach the desiredleakage current was about 35 minutes. The foil was discharged to 0 Vafter the second reformation process.

Subsequently, a third reformation of the oxide layer was performed inthe second aqueous solution at a target voltage of 480 V and the sametarget current as the second reformation process. The leakage currentafter the third reformation process reached 0.44 mA/cm². The approximatetime for the third reformation process to reach the desired leakagecurrent was about 10 minutes. The foil was discharged to 0 V after thethird reformation process. Subsequently, a fourth reformation of theoxide layer was performed in the second aqueous solution at a targetvoltage of 480 V and the same target current as the third reformationprocess. The leakage current after the fourth reformation processreached 0.41 mA/cm². The approximate time for the fourth reformationprocess to reach the desired leakage current was about 10 minutes. Thefoil was discharged to 0 V after the fourth reformation process.

A discharge capacitance test was performed on the foil after the fourthreformation process whereby a voltage of about 50 V was applied betweenthe immersed foil and the second aqueous solution, and a change incurrent was monitored over time. The capacitance of the foil wasdetermined to be 1.22 μF/cm².

The foil was removed from the second solution and immersed in aphosphate solution for 2 minutes. The phosphate solution was an aqueoussolution having 2 wt % ammonium dihydrogen phosphate. The foil was thenremoved from the phosphate solution and rinsed for 2 minutes with water.

A number of capacitors were manufactured using the anode having theoxide layer prepared above. Each capacitor had 29 anodes, 9 cathodeshaving Ti as the cathode material, and used ethylene glycol as theelectrolyte.

FIG. 3 depicts an SEM image of the oxide layer which is predominantly aboehmite phase. In comparison to a pseudo-boehmite phase (shown in FIG.5), the boehmite phase is more dense and less porous. FIG. 4 depicts thepercent deformation of the capacitor under accelerated aging conditions.The percent deformation is equal to the time to charge the capacitor atthe end of a measurement period (ti) minus the time to charge thecapacitor at time zero (to) divided by the charge time at to. Forexample, to measure the percent deformation after a period of 9 months,one measures the charge time at 0 months and the charge time at 9months.

The percent deformation may be measured using an accelerated agingprocess. In such process, the capacitor is heated under atmosphericpressure for 50 hours at 90° C., which is equivalent to 9 months at 37°C. The other measurements scale proportionally. For example, thecapacitor is heated for 132 hours at 90° C. for an equivalent aging of24 months at 37° C. The data at each age in FIG. 4 is the result ofabout 20 to 30 capacitors being measured. As shown in FIG. 4, the methodof Example 1 results in consistently reproducible results at each age.

Comparative Example 1

In Comparative Example 1, an anode having an oxide layer was prepared inaccordance with a process that uses several heating steps. The anode wasassembled into a electrolytic capacitor and deformation was measuredusing an accelerated aging process.

An aluminum foil having the same specifications as that of Example 1 wasplaced in a water bath for 8 minutes at 95° C. to form a hydrationlayer. The aluminum foil was removed from the water bath and placed in afirst aqueous solution having 0.5 wt % citric acid. An oxide layer wasformed on the aluminum foil using a target voltage of 490 V and a targetcurrent set point of 14.8 mA/cm². The formation process continued at thetarget current until the target voltage was reached, and then the targetvoltage was maintained and the current was allowed to drop. The currentdropped until the leakage current reached 1.11 mA/cm². The time forforming the oxide layer was about 160 minutes. After the desired leakagecurrent was reached, the voltage was lowered to about 50 V, and then thefoil was discharged to 0 V before proceeding to the next step.

After the formation process, the anode foil having the reformed oxidelayer was removed from the first aqueous solution and heated in aLinberg Blue M oven at atmospheric pressure conditions for 4 minutes at500° C.

The heated anode foil was removed from the oven and placed in an aqueoussolution having 0.5 wt % citric acid. This aqueous solution is the sameas that used for the formation step. A first reformation of the oxidelayer was performed using a target voltage of 485 V and the same targetcurrent as the formation process. The leakage current after the firstreformation process reached 0.74 mA/cm². The time for the firstreformation process to reach the desired leakage current was about 50minutes. After the desired leakage current was reached, the foil wasdischarged to 0 V in the same manner as the formation step.

After the first reformation process, the anode foil having the reformedoxide layer was removed from the aqueous solution and heated in the ovenat atmospheric pressure conditions for 4 minutes at 500° C.

The heated anode foil was removed from the oven and placed in an aqueoussolution having 0.5 wt % citric acid. A second reformation of the oxidelayer was performed using a target voltage of 480 V and the same targetcurrent as the first reformation process. The leakage current after thesecond reformation process reached 0.55 mA/cm². After the desiredleakage current was reached, the foil was discharged to 0 V in the samemanner as discussed for prior steps.

After the second reformation process, the anode foil having the reformedoxide layer was removed from the second aqueous solution and heated inthe oven at atmospheric pressure conditions for 4 minutes at 500° C.

The heated anode foil was removed from the oven and placed in an aqueoussolution having 0.5 wt % citric acid. A third reformation of the oxidelayer was performed using a target voltage of 480 V and the same targetcurrent as the second reformation process. The leakage current after thethird reformation process reached 0.55 mA/cm². After the desired leakagecurrent was reached, the foil was discharged to 0 V in the same manneras discussed for prior steps.

After the third reformation process, the anode foil having the reformedoxide layer was removed from the aqueous solution and heated in the ovenat atmospheric pressure conditions for 4 minutes at 500° C.

The heated anode foil was removed from the oven and placed in an aqueoussolution having 0.5 wt % citric acid. A fourth reformation of the oxidelayer was performed using a target voltage of 480 V and the same targetcurrent as the third reformation process. The leakage current after thefourth reformation process reached 0.55 mA/cm². After the desiredleakage current was reached, the foil was discharged to 0 V in the samemanner as discussed for prior steps.

A discharge capacitance test was performed on the foil after the fourthreformation process whereby a voltage of about 50 V was applied betweenthe immersed foil and the second aqueous solution, and a change incurrent was monitored over time. The capacitance of the foil wasdetermined to be 1.18 μF/cm². The capacitance of the foil in ComparativeExample 1 is about 3% lower than the foil of Example 1.

The foil was removed from the second solution and immersed in aphosphate solution for 2 minutes. The phosphate solution was an aqueoussolution having 2 wt % ammonium dihydrogen phosphate. The foil was thenremoved from the phosphate solution and rinsed for 2 minutes with water.

A number of capacitors were manufacturing using the foil having theoxide layer prepared by the methods of Comparative Example 1. Eachcapacitor was prepared in the same manner as Example 1.

FIG. 5 depicts an SEM image of the oxide layer which is predominantly apseudo-boehmite phase. FIG. 6 depicts the percent deformation of thecapacitor under the same accelerated aging conditions used in Example 1.As shown in FIG. 6, the deformation of the capacitor in ComparativeExample 1 is about 20% at 36 months, and reaches a deformation of about30% or more at 72 months. In contrast, the deformation in the capacitorof Example 1 is lower. For instance, the deformation is about 10% orless for up to 51 months and remains at less than 20% after 177 months.The deformation in Example 1 is reduced about 65% in comparison to thatin Comparative Example 1. Moreover, the variation in deformation isreduced using the method of Example 1 as well. Comparative Example 1,which shows a large variation at 9 months, is problematic because it isa period just prior to the maintenance cycle. The large variationindicates that capacitors formed by the method of Comparative Example 1are not reliable in the period just prior to the maintenance cycle andcould potentially fail if needed during this period. Reducing thedeformation can result in increased ICD longevity due to less need forcapacitor maintenance cycles. Accordingly, the time between eachcapacitor maintenance cycle can be increased. It is estimated that thecapacitor of Example 1 will not require a maintenance cycle for at least24 months, which exceeds the requirement of the capacitor of ComparativeExample 1 by more than 1 year.

To summarize, the present disclosure describes a method of producing anoxide layer on an anodic foil for use in a capacitor. The methodincludes immersing an anodic foil in a first solution; maintaining atarget current between the immersed anodic foil and the first solutionuntil a first target voltage is reached to form an oxide layer overlyingthe anodic foil; discharging the immersed anodic foil after the firsttarget voltage is reached; maintaining the target current between theimmersed anodic foil and the first solution until a second targetvoltage is reached to reform the oxide layer a first time; dischargingthe immersed anodic foil after the second target voltage is reached;removing the anodic foil from the first solution and heating the anodicfoil at a temperature sufficient to induce defects in the reformed oxidelayer; immersing the heat treated anodic foil in a second solution;maintaining the target current between the immersed anodic foil and thesecond solution until a third target voltage is reached to reform theoxide layer a second time; and discharging the immersed anodic foilafter the third target voltage is reached; and/or

maintaining the target current between the immersed anodic foil and thesecond solution until a fourth target voltage is reached to reform theoxide layer a third time; and discharging the immersed anodic foil afterthe fourth target voltage is reached; and/or

the fourth target voltage may be maintained until a desired leakagecurrent between the immersed anodic foil and the second solution isachieved, and the method may include removing the anodic foil from thesecond solution and heating the anodic foil a second time at atemperature sufficient to induce defects in the reformed oxide layerwhen the desired leakage current is not achieved during maintenance ofthe fourth target voltage; and/or

the step of heating the anodic foil a second time may be performed at atemperature of between about 200° C. and about 400° C. and for a timeperiod of between about 30 seconds and about 6 minutes; and/or

the method may further include maintaining a target current between theimmersed anodic foil and the second solution until a fifth targetvoltage is reached to reform the oxide layer a fourth time; anddischarging the immersed anodic foil after the fifth target voltage isreached; and/or

the method may further include removing the anodic foil from the secondsolution; immersing the anodic foil in a phosphate-containing solution;removing the anodic foil from the phosphate-containing solution; andrinsing the anodic foil; and/or

the first target voltage may be greater than the second target voltage,the second target voltage may be greater than the third target voltage,and the third, fourth and fifth target voltages may be equal; and/or

a difference between the first target voltage and the second targetvoltage may be about 5 Volts (V), and a difference between the secondtarget voltage and the third, fourth and fifth target voltages may beabout 5 V; and/or

the first target voltage may be maintained until a first desired leakagecurrent is achieved; and/or

the second target voltage may be maintained until a second desiredleakage current is achieved, and the third target voltage may bemaintained until a third desired leakage current is achieved; and/or

the first target voltage may be greater than the second target voltage,and the second target voltage may be greater than the third targetvoltage; and/or

the method may further include hydrating the anodic foil to form a waterhydration layer thereon prior to immersing the anodic foil in the firstsolution; and/or

the first and second solutions may include an ionogen; and/or

the first and second solutions may include a phosphate; and/or

the heating step may be performed at a temperature of between about 350°C. and about 650° C. and for a time period of between about 30 secondsand about 6 minutes; and/or

the anodic foil may include aluminum and the oxide layer may be analuminum oxide; and/or

the oxide layer may include a boehmite phase and a pseudo-boehmitephase, and, after the oxide layer has been reformed the second time, theamount of the boehmite phase may be greater than the amount of thepseudo-boehmite phase.

Also described herein is a device including a conductive anode; a layerof a barrier oxide disposed on a surface of the conductive anode,wherein the barrier oxide is an aluminum oxide having a boehmite phaseand a pseudo-boehmite phase, the amount of the boehmite phase beinggreater than the amount of the pseudo-boehmite phase; a conductivecathode; a separator disposed between the anode and the cathode; and anelectrolyte disposed between the anode and the cathode; and/or

the device may be an electrolytic capacitor.

Although the disclosure herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent disclosure. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present disclosure as defined by the appended claims.

The invention claimed is:
 1. A method of producing an oxide layer on ananodic foil for use in a capacitor, the method comprising: immersing ananodic foil in a first solution; maintaining a target current betweenthe immersed anodic foil and the first solution until a first targetvoltage is reached to form an oxide layer overlying the anodic foil;discharging the immersed anodic foil after the first target voltage isreached; maintaining the target current between the immersed anodic foiland the first solution until a second target voltage is reached toreform the oxide layer a first time; discharging the immersed anodicfoil after the second target voltage is reached; removing the anodicfoil from the first solution and heating the anodic foil at atemperature sufficient to induce defects in the reformed oxide layer;immersing the heat treated anodic foil in a second solution; maintainingthe target current between the immersed anodic foil and the secondsolution until a third target voltage is reached to reform the oxidelayer a second time; and discharging the immersed anodic foil after thethird target voltage is reached.
 2. The method of claim 1, furthercomprising: maintaining the target current between the immersed anodicfoil and the second solution until a fourth target voltage is reached toreform the oxide layer a third time; and discharging the immersed anodicfoil after the fourth target voltage is reached.
 3. The method of claim2, wherein the fourth target voltage is maintained until a desiredleakage current between the immersed anodic foil and the second solutionis achieved, the method further comprising: removing the anodic foilfrom the second solution and heating the anodic foil a second time at atemperature sufficient to induce defects in the reformed oxide layerwhen the desired leakage current is not achieved during maintenance ofthe fourth target voltage.
 4. The method of claim 3, wherein the step ofheating the anodic foil a second time is performed at a temperature ofbetween about 200° C. and about 400° C. and for a time period of betweenabout 30 seconds and about 6 minutes.
 5. The method of claim 2, furthercomprising: maintaining a target current between the immersed anodicfoil and the second solution until a fifth target voltage is reached toreform the oxide layer a fourth time; and discharging the immersedanodic foil after the fifth target voltage is reached.
 6. The method ofclaim 5, further comprising: removing the anodic foil from the secondsolution; immersing the anodic foil in a phosphate-containing solution;removing the anodic foil from the phosphate-containing solution; andrinsing the anodic foil.
 7. The method of claim 5, wherein the firsttarget voltage is greater than the second target voltage, the secondtarget voltage is greater than the third target voltage, and the third,fourth and fifth target voltages are equal.
 8. The method of claim 7,wherein a difference between the first target voltage and the secondtarget voltage is about 5 Volts (V), and a difference between the secondtarget voltage and the third, fourth and fifth target voltages is about5 V.
 9. The method of claim 1, wherein the first target voltage ismaintained until a first desired leakage current is achieved.
 10. Themethod of claim 1, wherein the second target voltage is maintained untila second desired leakage current is achieved, and the third targetvoltage is maintained until a third desired leakage current is achieved.11. The method of claim 1, wherein the first target voltage is greaterthan the second target voltage, and the second target voltage is greaterthan the third target voltage.
 12. The method of claim 1, furthercomprising: hydrating the anodic foil to form a water hydration layerthereon prior to immersing the anodic foil in the first solution. 13.The method of claim 1, wherein the first and second solutions include anionogen.
 14. The method of claim 13, wherein the first and secondsolutions include a phosphate.
 15. The method of claim 1, wherein theheating step is performed at a temperature of between about 350° C. andabout 650° C. and for a time period of between about 30 seconds minutesand about 6 minutes.
 16. The method of claim 1, wherein the anodic foilincludes aluminum and the oxide layer is an aluminum oxide.
 17. Themethod of claim 16, wherein the oxide layer includes a boehmite phaseand a pseudo-boehmite phase, and, after the oxide layer has beenreformed the second time, the amount of the boehmite phase is greaterthan the amount of the pseudo-boehmite phase.